RU2210785C2 - Digital frequency meter - Google Patents

Abstract

FIELD: measurement technology, radio and electrical engineering, metrology and other branches of industry where precision measurement of frequency and its deviation from nominal value is required. SUBSTANCE: digital frequency meter includes reception port of input signal, first counter fitted with first reading register, reception port of pulse of measurement period, first synchronization circuit through which reception port of pulse of measurement period is connected to control input of first reading register, reference generator forming reference pulses, second counter provided with second reading register, processing and indication aid, inverter and second synchronization circuit. Reception port of input signal is connected through second synchronization circuit to clock input of first counter, reference generator is connected to clock input of second counter, clock input of second synchronization circuit and through inverter to clock input of first synchronization circuit. Input of control over second reading register is connected to output of first synchronization circuit, outputs of counters are connected through reading registers to processing and indication aid. Invention can be employed to obtain statistic parameters characterizing stability of frequency of signals in various time periods. EFFECT: elimination of dead time periods characteristic of digital frequency meters, increased precision of measurement of signals during little time intervals, widened range of measured frequencies. 3 cl, 12 dwg

Description

 The invention relates to measuring equipment and can be used in radio engineering, electrical engineering, metrology and other industries for precision measurement of signal frequency, frequency deviations from the nominal value, time intervals, and also to obtain statistical parameters characterizing the stability of the signal frequency over different periods of time.

Digital frequency meters of various designs are known. For example, a frequency meter is known, including: an input signal receiving port containing input means and a pulse shaper, a first counter, a selector stage, a measuring period shaper containing a reference generator, a second counter and control means, as well as processing and indication [Electro-radio measurements. / V.I. Vinokurov, S.I. Kaplin, I.G. Petelin - M .: "Higher school.", 1986, p. 148-160]. This frequency meter works as follows (figure 1). The input signal is received through the input signal receiving port and through the selector stage to the first counter. An input signal receiving port containing input means and a counting pulse generator converts the measured signal into a sequence of rectangular counting pulses F _I , whose front coincides with the time the input signal passes through the zero phase in a given direction (for example, in the positive). The first counter counts the number of these pulses for the time determined by the shaper of the measuring period. For this purpose, the shaper of the measuring period generates a pulse F _G , which opens the selector stage for the duration of the measuring period, and after a while generates a “reset” pulse, resetting the first counter to start a new measurement. The shaper of the measuring period contains a series-connected reference generator, a second counter and control means. The frequency from the output of the reference generator is divided by a second counter. As a result, an impulse of a given duration is formed. The selector stage is open only for the duration of the pulse F _G , while the first counter receives counting pulses received from the input signal. The first counter calculates their number for the measurement period. The output of the first counter data is connected to the processing and indication means, which contains a decoder with a digital indicator that allows you to observe the code calculated as a result of the count and an interface for connecting a computer. At the end of the reading period, which is marked by the "reset" command, the counters are returned to their initial state, and a new measuring period begins, i.e. the period during which counting pulses F _I are counted during the existence of a new pulse F _G.

The period between the end of the pulse F _G and the beginning of the reset pulse is necessary for reading the second counter. It is called "dead" time, because at this time the selector stage does not pass counting pulses and the frequency measurement is interrupted. This drawback does not allow averaging several measurements and attributing the result of averaging to the combined interval. In addition, this increases the error in measuring the statistical characteristics of frequency stability.

Another disadvantage of this frequency meter is that when the beginning or end of the pulse F _G and the beginning of one of the counting pulses F _I (generated from the input signal) are close in time, this pulse can either be counted or not counted, which creates an ambiguity in the measurement result.

Another digital frequency meter is also known [US Pat. No. 4,984,254, FIG. 1], comprising: a port for receiving a pulse of a measuring period, a port for receiving an exemplary signal, a synchronization circuit, a counter, a read register (FIG. 2). Frequency measurement is carried out by counting the number of edges of the input signal during the pulse duration of the measuring period. The synchronization circuit generates a modified pulse of the measuring period F _G1 , whose edges are offset in time so as to coincide with the edges of the counting pulses F _I generated from the input signal. There is no ambiguity in the counting result in this digital frequency meter, since the counting time and the reading time are separated by a synchronization circuit. However, between the pulses of the measuring period, the operation of the frequency meter, as well as the above, is interrupted to read the results, i.e. "dead time" comes. This frequency meter is the closest analogue of the proposed for the largest number of similar features and is taken as a prototype of the invention. Its disadvantage is the presence of "dead time".

 The problem to which the invention is directed, is the creation of a frequency meter that can operate continuously without "dead time".

 The problem is solved by the fact that a digital frequency meter is proposed, including an input signal receiving port, converting the input signal into a sequence of counting pulses, a first counter equipped with a first reading register, a pulse receiving port for the measuring period and a first synchronization circuit through which the output of the pulse receiving port for the measuring period connected to the control input of the first read register, an exemplary generator generating exemplary pulses, a second counter provided with a second read register, cf processing and indication, as well as an inverter and a second synchronization circuit, while the input signal receiving port is connected via a second synchronization circuit to the clock input of the first counter, the reference generator is connected: to the clock input of the second counter, with the clock input of the second synchronization circuit and through the inverter with the clock input of the first synchronization circuit, the control input of the second read register is connected to the output of the first synchronization circuit, and the outputs of each counter are connected through the counter reading registers with the means of boots and indications. The synchronization circuits are designed in such a way that in response to each input pulse an output pulse is generated, the front of which coincides with the leading edge of the pulses supplied to their clock inputs (Fig. 3).

 The operation of the digital frequency meter is as follows.

По окончании каждого измерительного периода, то есть с приходом нового фронта импульсов точных границ WR, счетчики не останавливаются и не обнуляются, а продолжают считать. Это не препятствует чтению данных из них в регистры, поскольку процессы счета и чтения разделены во времени на половину периода образцовой частоты F_O. Это достигается тем, что, благодаря первой схеме синхронизации, фронты импульсов F_IS и F_O, подаваемых на тактовые входы первого и второго счетчиков, привязаны во времени к передним фронтам образцовых импульсов F_O, а, благодаря второй схеме синхронизации с инвертором на тактовом входе, фронты импульсов точных границ WR привязаны во времени к задним фронтам образцовых импульсов F_O. На входе второго счетчика схема синхронизации не требуется, поскольку на его вход поступают сами образцовые импульсы F_O.The input signal receiving port converts the measured signal F _BX into a sequence of counting pulses F _I whose edges coincide with the time the input signal F _BX passes through the zero phase in each direction. The input signal is received through the input signal receiving port and through the first synchronization circuit to the first counter. An exemplary generator generates exemplary pulses F _{O of a} given frequency, the magnitude of which is greater than the frequency of the input signal F _BX . The first synchronization circuit from each counting pulse F _I generates a synchronized pulse F _IS , the front of which coincides with the leading edge of the nearest next reference pulse F _O from the reference generator. The first counter increases the accumulated code with the arrival of each synchronized pulse F _IS , and the second counter increases the code with the arrival of each reference pulse F _O. The port for receiving the pulse of the measuring period receives pulses F _G , the edges of which mark the preparation of the end of the current measuring period and the beginning of the next. These pulses are fed to the second synchronization circuit, which generates the exact edges of the signal WR along the trailing edge of the nearest reference pulse F _O , which controls the reading of codes from both counters into their read registers. Thus, the leading edges of the pulses WR mark the exact boundaries of the measurement period. Codes K _{1 (i)} and k _{2 (i)} , read at moments t _i from the counters in their registers, are input to the processing and indication means, which is implemented on a computer. In this tool, the average frequency for each measuring period is determined as the product of the known model frequency F _O and the ratio of the increment of the code of the second counter ΔK ₂ = K _{2 (i + 1)} -K _{2 (i)} to the increment of the code of the first counter ΔK ₁ = K _{1 ( i + 1)} -K _{1 (i)} :

At the end of each measurement period, that is, with the arrival of a new pulse front of the exact boundaries of WR, the counters do not stop and do not reset, but continue to count. This does not preclude reading data from them into registers, since the counting and reading processes are divided in time by half the period of the reference frequency F _O. This is achieved by the fact that, thanks to the first synchronization circuit, the edges of the pulses F _IS and F _O supplied to the clock inputs of the first and second counters are tied in time to the leading edges of the reference pulses F _O , and, thanks to the second synchronization circuit with an inverter at the clock input , the pulse fronts of the exact boundaries WR are tied in time to the trailing edges of the model pulses F _O. At the input of the second counter, a synchronization circuit is not required, since the model pulses F _O themselves are supplied to its input.

 The capacity of each meter is selected so that it does not overflow during the measurement period. The counting result for each counter is determined by subtracting the previous code value from the subsequent code value. In the event of a counter overflow, the result of the subtraction becomes negative. Then, the counter capacity is added to this negative value, which restores its true value in this measurement cycle.

 Each of the two synchronization schemes can be performed, for example, in the form of a shear trigger, the D-input of which is the main input of the device, and the C-input is the clock input (figure 4). When the leading edge of the pulse arrives at the C input, the output signal becomes the same as the signal supplied to the D input. If at these moments the output signal coincides with the signal at the D-input, then it does not change its value. The operation diagrams of the synchronization circuit are shown in FIG. 5. As follows from the description, the frequency meter can operate continuously, without stopping to read the results, i.e. without dead time.

 The generator of the counting pulses of the frequency meter can be performed, for example, as described in the book of Electroradio measurements. / IN AND. Vinokurov, S.I. Kaplin, I.G. Petelin - M.: Higher School, 1986, p. 148-160. If the noise level is small, then the comparator or Schmitt trigger can serve as a shaper of counting pulses.

 Counters can be performed, for example, on a specialized chip KR1810VI1, which contains all the necessary elements and is turned on according to the standard scheme.

 Means of processing and display can serve, for example, computers.

 The port for receiving the pulse of the measuring period can be performed, for example, as a repeater or as a Schmitt trigger, and can also be replaced by a pulse shaper of the measuring period, for example, an electronic clock, including a clock included in the computer.

 An exemplary generator can be any generator whose accuracy is sufficient for the required measurement accuracy, for example, a conventional quartz oscillator, and for higher accuracy, a hydrogen or rubidium frequency standard. A reference generator may have an inverted auxiliary output. In this case, an additional inverter is not required to obtain an inverted signal.

 Additionally, the proposed frequency meter can solve the problem of increasing the accuracy of measurements when measuring for small time intervals. Typically, in a digital frequency meter, the measurement error is inversely proportional to the product of the measured frequency by the measuring interval.

Измерение кратковременных девиаций частоты F требует уменьшения измерительного периода до τ≤10^-3 с, что приводит к недопустимому росту ошибки дискретности δF_% при F≈10⁶ Гц. Для снижения δF_% требуется не только измерять целое число импульсов, но и определять дробное число импульсов измеряемой частоты, попавшее в измерительный период.The number of pulses ΔK ₁ = K _{1 (i + 1)} -K _{1 (i)} when calculating is always expressed as an integer, while the true value of the number of periods of the measured frequency throughout the measurement interval is always a fractional number, since the frequencies F _I and F _О not multiple. It follows that there is always an error in determining the number of counting pulses F _I during the time between subsequent leading edges of the pulses WR by an amount within unity. This error, referred to the total number of counted pulses, is called the discrete error. This value increases with decreasing measurement period. So, if the frequency F (Hz) is measured in a counting way for the measurement period τ (s), then the relative error δF _{% of the} measurement of the average frequency is:

The measurement of short-term frequency deviations F requires a decrease in the measuring period to τ≤10 ^-3 s, which leads to an unacceptable increase in the discrete error δF _% at F≈10 ⁶ Hz. To reduce δF _%, it is necessary not only to measure the integer number of pulses, but also to determine the fractional number of pulses of the measured frequency that fell into the measuring period.

The accuracy of the frequency meter measurements can be improved by determining the interval between the leading edges of the pulses of the exact boundaries WR and the leading edges of the counting pulses F _I.

 For this purpose, an error pulse former, a time scale converter, and a duration measurement channel are introduced into the frequency meter, the inputs of this former being connected to the outputs of the input signal receiving port and the second synchronization circuit, and the output of the duration measuring channel connected to the input of the processing and indication means (Fig. .6).

The simplest version of the error pulse shaper is, for example, a trigger with separate start, as shown in Fig.7. The pulse front of the exact boundaries WR sets the output signal ER to the active state, and the next counting pulse F _I returns it to its original state, thus forming the pulse duration ER. The operation diagrams of the error pulse shaper are shown in FIG.

 A time scale converter converts a short pulse into an extended pulse with a given conversion factor for the duration.

 The converter, for example, can be made of two current sources of opposite signs, one of which is switched, and the current of the switched source is a specified number of times (K + 1) higher than the current of the non-switched source. The source also contains a capacitor, a control circuit, and a threshold device. Functional diagram is shown in Fig.9. The operation diagrams of the circuit are shown in FIG. 10.

 The simplest version of the duration measurement channel is implemented, for example, by introducing a third counter equipped with a third register, included in the same way as the second counter with the second register, but the selector input of the third counter is the input of the duration measurement channel. An example of such a channel for measuring the duration is shown in Fig.6 in the dotted area. The selector input of the third counter allows you to stop the count at the end of the pulse at the output of the stretching circuit. Since the third counter stops the count before reading its code, the use of a register at its output is not necessary. Many counters have this register at the output as part of the microcircuit, therefore this register is in the preferred structure.

.The device operates in the same way as the digital frequency meter described above, determining the values of the new values of the codes of the first and second counters K _{1 (i)} and K _{2 (i)} obtained as a result of counting the counting pulses F _I and reference pulses F _O for the measuring period. In addition to this, the error pulse shaper generates a short pulse ER, beginning with the beginning of the pulse of the exact boundaries WR and ending with the beginning of the next counting pulse F _I. The ER error pulse, coming to the stretching circuit, generates an EL pulse, the duration of which is longer than the input pulse by a predetermined number M times (M≈1000). The duration of the error pulse measures the duration measurement channel, and the result enters the processing and indication means. To this end, the third counter considers the number of model pulses F _O received at its input during the duration of the stretched pulse EL. For this, an EL pulse is applied to the selector input of this counter. The accumulated code K _{3 (i)} , equal to this number of reference pulses F _O , at the end of the measurement interval, as well as the codes K _{1 (i)} and K _{2 (i)} , is fed to the processing and indication means. The processing and display means calculates the adjusted value K _{Y1 (i)} for K _{1 (i)} according to the formula

.

.From the obtained adjusted values of K _{Y1 (i), the} adjusted values of the frequency F _Y averaged over the i-th measurement intervals are calculated:

.

In this case, the accuracy of determining the value of K _{Y1 (i)} is M times higher. Since the duration of the pulses WR, marking the exact boundaries of the measuring period, is an integer number of periods of the reference frequency, the number K _{2 (i) is} always strictly integer. Therefore, increasing the accuracy of determining the value of K _{Y1 (i)} allows to increase the accuracy of measuring the average frequency in the measuring interval by the same amount.

Таким образом, в предлагаемом цифровом частотомере дополнительно решена задача увеличения точности измерений (в М≈1000 раз).The accuracy limit for this frequency meter is determined by the relation

Thus, the proposed digital frequency meter additionally solved the problem of increasing the accuracy of measurements (M ≈ 1000 times).

откуда получаем минимальную измеряемую частоту, равную обратной величине:

Для расширения диапазона частот частотомер снабжается вторым формирователем импульса ошибки с вторым каналом измерения длительности на выходе и устройством селекции несовпадения, причем входы второго формирователя соединены с выходами первой и второй схем синхронизации, выход второго канала измерения длительности соединен с входом средства обработки и индикации, первый формирователь импульса ошибки подключен к своему каналу измерения длительности через устройство селекции несовпадения, второй выход которого соединен с выходом второго формирователя импульса ошибки. Второй канал измерения длительности идентичен первому каналу, то есть содержит четвертый счетчик и четвертый регистр, включенные аналогично. Схема этого частотомера показана на фиг.11, а диаграммы работы - на фиг.12.In addition to these tasks, the problem of expanding the range of measured frequencies can be solved. The limitation on this range is due to the fact that the duration τ _{ER of the} extended pulse EL should not exceed the duration τ _{WR of the} pulse of the exact boundaries WR. For example, at τ _WR = 1 ms, M = 1000, the duration τ _{ER of the ER} pulse should satisfy the relation

whence we get the minimum measured frequency equal to the reciprocal:

To expand the frequency range, the frequency meter is equipped with a second error pulse shaper with a second output duration measuring channel and a mismatch selection device, the inputs of the second shaper connected to the outputs of the first and second synchronization circuits, the output of the second duration measuring channel connected to the input of the processing and indication means, the first shaper the error pulse is connected to its duration measurement channel through a mismatch selection device, the second output of which is connected to the w output cerned error pulse shaper. The second channel for measuring the duration is identical to the first channel, that is, it contains the fourth counter and the fourth register included in the same way. The circuit of this frequency meter is shown in FIG. 11, and the operation diagrams in FIG. 12.

The device works in the same way as described above, determining the values of the new values of the codes of the first and second counters K _{1 (i)} and K _{2 (i)} obtained as a result of counting the counting pulses F _I and reference pulses F _O for the measuring period. The first error impulse generator generates an ER error impulse, starting at the beginning of the exact boundaries impulse WR and ending at the beginning of the next counting impulse F _I. The second error pulse shaper generates a pulse of the integer part of the error ER2, starting at the beginning of the pulse of exact boundaries WR and ending at the beginning of the next synchronized pulse F _IS . The mismatch selection device generates a difference pulse ERD from the pulses ER and ER2, which takes a single value only for the time until the input pulses coincide. Since both pulses start simultaneously, the beginning of the ERD pulse coincides with the end of the ER pulse, and the end of the ERD pulse coincides with the end of the ER2 pulse. Thus, the difference pulse ERD arriving at the stretching circuit starts at the moment of the leading edge of the counting pulse F _I , which is next after the edge of the signal of exact boundaries, and this pulse ends ERD at the leading edge of the next synchronized pulse F _IS . Therefore, the duration of the difference pulse ERD never exceeds the period of the reference frequency F _O. This ERD pulse through the stretching circuit enters the first channel for measuring the duration, and the pulse ER2 enters the second channel for measuring the duration. The second channel measures the duration of an integer number of periods of the reference frequency, which is contained during the error pulse ER. To this end, the pulse ER2 opens the selector input of the fourth counter at this time. Since the duration of the measured interval strictly corresponds to an integer number of periods of the reference frequency F _O , the measurement of this duration by the counting method is ensured with high accuracy. The first channel measures the remainder from subtracting from this pulse the ER errors of the indicated integer part ER2. To do this, the indicated ERD residue is fed to the stretching circuit, and from it the stretched pulse EL is supplied to the selector input of the third counter.

The duration τ _{ERD of an ERD} pulse, irrespective of the magnitude of the measured frequency, never exceeds the duration of one period of the reference frequency, so it can be stretched a large number of times. For example, at F _O = 8 • 10 ⁶ Hz and at τ _ERD = 1 ms, the upper value of the stretching coefficient M is limited to M≤8000, which allows a sufficiently high accuracy in measuring the duration of this interval.

Таким образом, описанный частотомер дополнительно решает задачу осуществления измерений частоты сигналов в большом диапазоне частот высокой точностью за малый измерительный период.In this regard, restrictions on the minimum value of the frequency are set based on the requirement that during the measurement period there should be at least two to three whole periods of the measured frequency. For example, with τ _WR = 1 ms, M = 1000, the minimum measured frequency is

Thus, the described frequency meter additionally solves the problem of measuring the frequency of the signals in a large frequency range with high accuracy for a small measuring period.

Claims (4)

 1. A digital frequency meter including an input signal receiving port that converts the input signal into a sequence of counting pulses, a first counter equipped with a first read register, a pulse receiving port for the measuring period and a first synchronization circuit through which the output of the pulse receiving port for the measuring period is connected to the control input first reading register, characterized in that it contains an exemplary generator that generates exemplary pulses, a second counter provided with a second reading register, processing means and indications, as well as an inverter and a second synchronization circuit, wherein the input signal receiving port is connected via a second synchronization circuit to the clock input of the first counter, the model generator is connected to the clock input of the second counter, to the clock input of the second clock and through the inverter with the clock input of the first synchronization circuits, the control input of the second read register is connected to the output of the first synchronization circuit, and the outputs of each counter are connected through their respective read registers to the processing means and indication.  2. The frequency meter according to claim 1, characterized in that it comprises serially connected error pulse shaper, a time scale converter and a duration measurement channel, wherein the first and second inputs of the error pulse shaper are connected to the output of the first synchronization circuit and to the output of the input signal receiving port, and the output of the duration measurement channel is connected to the input of the processing and indication means.  3. The frequency meter according to claim 2, characterized in that the duration measurement channel includes a third counter provided with a third register, the selector input of the third counter being the input of the duration measurement channel, and the clock input of the third counter connected to the output of the reference generator.  4. The frequency meter according to claim 2 or 3, characterized in that it is equipped with a second error pulse shaper, a second duration measurement channel and a mismatch selection circuit, wherein the first and second inputs of the second error pulse shaper are connected to the output of the first and second synchronization circuits, respectively, its the output through the channel for measuring the duration is connected to the input of the processing and indication means, and the mismatch selection circuit is connected between the first error pulse shaper and the first channel for measuring the duration, the second the course of this circuit is connected to the output of the second error pulse former.

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